The instant invention is directed to improved battery separator systems suitable for use in lithium batteries. One aspect of the current invention formulation (A) provides a microporous separator extremely high in air permeability of less than 20 sec/10 cc, preferably less than 10 sec/10 cc for better performance (lowest impedance) for consumer applications and also relates to a method for producing the same. Another aspect of the current invention (formulation B) provides a microporous separator with low shutdown temperatures, (less than 110 degrees C.), melt integrity of more than 165 degrees C., preferably more than 200 degrees C. and to meet the safety requirements for HEV (Hybrid Electric Vehicle) applications.
The separators of the current invention will have applications in alkaline battery chemistries if subjected to surface modification, such as hydrophilic treatment by plasma irradiation, impregnation with a surface active agent, surface grafting, etc. In addition to batteries, the products produced by the current invention can be used in various fields of art. They will have applications in air filtration, water purification (a filter for separating microorganisms and viruses from water) and size exclusion.
A single layer separator of formulation (A) with extremely high air permeability, adequate shutdown behavior, good puncture resistance, high melt integrity, very low in shrinkage and low manufacturing costs has advantages over commercially available separators for consumer lithium ion batteries.
A separator with the above characteristics that can shutdown at approximately 135 degrees C. and can keep electrodes apart at temperatures above 165 degrees C. is highly desirable for use in consumer applications.
A single layer separator of formulation (B) with a low shutdown temperature and a high melt integrity has major advantages over commercially available multi-layer and single layer shutdown separators. In the prior art shutdown separators use two or three layers of membranes with different melt temperatures laminated together. They typically use a micro-porous high-density polyethylene (HDPE) shutdown layer laminated (or attached by other means) to one or two supporting layers. These supporting layers are usually made from a polypropylene substrate, either microporous or non-woven. Therefore, the shutdown temperature will be dictated by the melting point of the HDPE inner layer, which is 135 degrees C., with the melt integrity based on the melting point of the porous polypropylene layer (165 degrees C.). Commercially available single layer separators made from HDPEs have melt integrity problems, since they melt down at temperatures above 135 degrees C.
Lithium ion cells have two to three times higher energy density than nickel metal hydride batteries used in the current HEV's. Due to this high energy density of lithium ion batteries, auto makers are eager to replace the currently used nickel metal hydride battery packs in HEV's with a high energy density lithium ion battery pack.
Thus far, safety concerns, primarily the thermal run away characteristics of lithium ion batteries, have been a major obstacle impeding the use of lithium ion batteries in HEV applications. Thermal run away in lithium ion cells may occur due to system failures or accidents, causing the cell's internal temperature to reach over 100 degrees C. Unchecked, it will continue to rise to the point of fire and explosion.
A separator that can prevent this thermal run away is highly desirable for lithium ion batteries used in the HEV's. It should have a shutdown temperature of about 95-110 degrees C. Together with a low shutdown temperature the separator must itself maintain its integrity at elevated temperatures, so that it does not fail and allow the battery electrodes to short circuit. For the present application the separator should also have a melt integrity of above 165 degrees C., and preferably above 200 degrees C.
In prior art, in order to cope with both the shutdown and melt integrity issues, the separator system has been usually combined with one or two supportive layers. An HDPE microporous layer was primarily used as the shutdown layer. However HDPE will melt down at about 135 degrees C. U.S. Pat. No. 5,922,492 teaches making a battery separator comprising a pure polymeric, unfilled microporous polyolefin membrane, usually polyethylene based, laminated to a polyolefin, non-woven fabric, usually a polypropylene supportive layer. This combination achieves shutdown requirements needed for lithium ion cells.
The above prior-art separator does not provide higher temperature structural integrity needed to keep the electrodes separated at high temperatures. However, separators made with inert fillers are able to maintain their structural integrity at high temperatures without utilizing a supportive layer, thus keeping the electrodes separated under these condition.
U.S. Pat. Nos. 5,565,281, 5,922,492, 6,096,213 and 6,180,280 all discuss lamination of different microporous polymeric layers for achieving different properties such as shutdown behavior and puncture resistance. But Using two layers bonded together is costly and difficult to manufacture.
U.S. Pat. No. 6,562,519 introduces a single layer microporous film. However, it uses a cross-linking approach to improve the melt integrity and does not contain any filler. Cross-linking procedures usually use a polymer with a double bond in their chain as a cross-linking agent and cross-linking is usually done by the application of heat, and ultraviolet and electron beam bombardment. These procedures are usually expensive, the process is time consuming, and difficult. It is, therefore, not suitable for a low-cost separator production.
U.S. Pat. No. 4,650,730 teaches how to make a shutdown separator by attaching two or three layers of micro-porous sheets to achieve shutdown behavior. The three-layer approach suffers the same disadvantages as the two layer approach, as would be expected.
The current invention uses well-known methods and readily available materials, and subjects them to well-known processes as described in the following paragraphs.
UHMWPE's have superb chemical resistance, high tensile strength, high melt integrity (they do not have a melt index) and excellent pore forming characteristics suitable for battery separator applications.
Inert fillers are also used in the production of battery separators, primarily for achieving better pore structures, creating added tortuosity and increased porosity. However, fillers can also add properties such as structural integrity (high puncture resistance), reduced shrinkage, improved thermal stability, and fire retardation, and they keep the electrodes separated at high temperatures. Examples are filled polymeric sheets such as those described in U.S. Pat. Nos. 3,351,495, 4,287,276, and U.S. Pat. No. 6,372,379 (by current authors), in which, the electrolyte passes through the separator's microporous channels.
LMWPE's, LDPE's and LLDPE's are known in the prior art as shutdown polymers. They have lower melt temperatures than HDPEs and have been used as additives to UHMWPE or HDPE to reduce the shutdown temperatures of the membrane separators.
The current invention utilizes a commonly used prior art process, widely used for producing battery separators for lead acid and in some alkaline batteries, for lithium ion cells. The prior art process starts with the mixing and extruding a polymer and filler (i.e., TiO2 or Silica) with a plasticizer oil at high temperature and pressure through a coat-hanger sheet die, followed by calendering the sheet, followed by the removal of the oil by solvent extraction and heat setting, creating a microporous sheet.
The process of the current invention for producing formulation (A) incorporates a stretching and heat setting step after solvent extraction. For formulation (B) the stretching step is added after calendering followed by solvent extraction and heat setting the film.